The present invention relates to a microthruster which provides a small unit of force and, more particularly, to a microthruster for use as propellant device for translating or rotating a microsatellite.
The use of extremely small thrust impulse bits in packages of sub-millimeter length scales is under consideration for a wide variety of applications. The small size has so far made it desirable to keep the construction simple by producing only a simple burst disk nozzle opening Upon opening a burst disk, the gas expels from the thrust chamber and produces an impulse. U.S. patent application Ser. No. 08/912,709 referred to above discloses an integrated pulsed propulsion and structural support system for a microsatellite employing a microthruster capable of being fabricated by batch processing similar to integrated microelectronics.
Aerospike thrust augmentation has been used in large scale thrusters for several applications in the aerospace community, but not in microthrusters. In particular, aerospike rocket engines have been built and tested at launch vehicle length scales, but not at microelectromechanical (MEMS) length scales. These large scale thrusters for launch vehicles provide several hundred thousand pounds or more of thrust in a continuous thrusting application. This is in contrast to the small unit of force associated with microthrusters, typically expressed in impulse units of micro to nano-pound-seconds, for example, which occurs only over an extremely short interval or blow-down time, less than or equal to 103 microseconds. The aerospikes in large scale thrusters have been formed as separate elements supported by struts attached to the thruster and are subject to a thermal management problem, e.g., heating.
There is a need for an improved microthruster that will provide higher thrust efficiency. A second need for development of thrust impulse bits is to make the magnitude of the impulse from microthrusters more uniform and controllable. This feature is valuable for design controllability when combining individual microthrusters to form arrays and for achieving minimum altitude dependence of thrust for either individual microthrusting or arrays of microthrusters. An object of the present invention is to provide an improved microthruster useful, for example, as a propellant for a microsatellite, capable of meeting these needs.
A microthruster according to the present invention comprises a closed chamber for carrying a fluid with a portion of the chamber less resistant to rupture from elevated fluid pressure than other portions of the chamber. The microthruster further includes an aerospike, which extends outwardly beyond the face of an outer wall of the closed chamber in the vicinity of the portion less resistant to rupture. The aerospike can be either axisymmetric or linear. It has been found that this microthruster of the invention improves the gas dynamics during propulsion for higher thrust efficiency and more controllable and uniform impulse characteristics. The microthruster is particularly useful as a propellant for a microsatellite.
In the disclosed embodiment of the invention, the aerospike is located between two adjacent diaphragms which close the chamber of the microthruster and form an exit section of the chamber which allows exit flow along the surface of the aerospike upon rupture of the diaphragms. It has been found that the introduction of the aerospike in the center of the exit section, opened by use of the two diaphragms in the form of two burst disks (or by an annular burst disk for an axisymmetric aerospike design), thereby allowing exit flow on each side of the aerospike, produces a higher impulse (19% more by calculation), as well as more uniform impulse performance over altitude variations, as compared with the microthrusters of application Ser. No. 08/912,709, which have no thrust augmentation. Preferably, the aerospike is formed integrally with the microthruster, Advantageously, it has been found that there is no thermal management (heating) problem with the aerospike in the microthruster of the invention.
The microthruster of the invention is preferably part of an array configuration comprising a plurality of the microthrusters stacked together, side-by-side, to provide numerous linear (or axisymmetric) impulse bits. The microthrusters of the inner array and those of the outer arrays in the array configuration can be exposed to different external pressure. However, because the bit impulse from the improved microthruster of the present invention is relatively pressure-independent, the array configuration with the microthruster of the present invention produces a more uniform thrust and hence is more controllable, as compared with an array configuration formed of microthrusters of the type disclosed in the aforementioned application Serial No. 08/912,709.